Emerging mobile network trends may call for denser deployments in urban areas, due to increasing traffic and/or user density. Operators are looking to deploy smaller cells (e.g., pico and/or femto cells) in the same areas as macro cells, with varying degrees of “cooperation” and/or integration between them according to the desired level of network performance (so-called “HetNet deployments” or heterogeneous network deployments). An issue in such deployments is to provide increased/optimum mobility to UEs (user equipment nodes, also referred to as wireless terminals) that have different capabilities while providing/ensuring a reduced/lowest level of core network (CN) involvement, to reduce signalling traffic.
In WCDMA (Wide Band Code Division Multiple Access) systems, a UE that supports Release 9 or later releases may be able to provide specific measurements to support mobility towards femto/pico cells. In particular, these UEs may be able to improve mobility towards closed HNB (Home Node B) cells, also known as CSG (Closed Subscriber Group) cells, by providing a Proximity Indication and specific System Information measurements that allow the serving RNC (Radio Network Controller) to disambiguate the target cell and to trigger the appropriate relocation procedures towards the right target. As used herein, the term Serving RNC (SRNC) can refer to both a HNB or an RNC. Moreover, a cell may be referred to as an eNodeB or a base station, an HNB cell may be referred to as an HNB or a home base station, and/or a CSG cell may be referred to as a CSG or a CSG base station. Moreover, an RNC may be implemented as a macro/network base station or an element thereof.
Information concerning specific measurements performed by Release 9 and later UEs can be found in the reference 3GPP TS 25.367 Rel-10 V10.0.0, “Mobility procedures for Home Node B (HNB),” Stage 2, Release 10, 2011-03, and/or in the reference 3GPP TS 25.331 V10.3.1, “Radio Resource Control (RRC); Protocol Specification,” Release 10, 2011-05. A graphical description/illustration of how measurement configuration and reporting occur for a Release 9 or later UE handing over to a CSG cell are shown in FIG. 1 below (extracted from the reference 3GPP TS 25.367, cited above).
In message 1 [1. MEASUREMENT CONTROL (Measurement Type=CSG Proximity Detection)], the SRNC configures the UE to report a CSG Proximity Indication flag. When reported, this flag indicates that the UE is close to an accessible CSG cell. In message 2 [MEASUREMENT REPORT (CSG Proximity Indication)], the UE reports the Proximity Indication flag due to having determined that it is in proximity of an accessible CSG cell.
CSG cells are restricted to use a limited range of PSCs (Primary Scrambling Codes). This is due to the PSC split applied between closed CSG cells and any other cell. Hence, due to the limited number of available PSCs and to the high number of closed CSG cells potentially present in a HetNet deployment, there may be a possibility that a PSC is reused between two or more CSG cells in the same neighbourhood.
For the reasons above, the SRNC configures measurements at the UE via message 3 [3. MEASUREMENT CONTROL (CSG Intra-frequency cell information, Intra-frequency SI Acquisition)]. This configuration is meant to make the UE measure the SIBs (System Information Blocks) of the CSG cell in proximity together with the PSC of such cell.
Once the UE performs the measurements, it will report the CSG cell information required by the SRNC in message 4 [4. MEASUREMENT REPORT (PSC, Cell Identity, CSG Member Indication)]. Such information includes the following:                (1) CSG cell's PSC,        (2) CSG cell's CSG ID (CSG Identification), identifying the closed subscribed group to which the CSG cell belongs,        (3) Cell Global Identity (CGI), uniquely identifying the cell in the PLMN (Public Land Mobile Network), and        (4) Membership Status (i.e., the membership of the UE in the detected CSG cell) where this parameter is provided according to the subscriber information the UE has provided at that point in time and it can be set to “member” or “non-member.”With the information above, the SRNC can determine whether the UE is allowed in the detected CSG cell, and it can unequivocally detect such cell by means of the CGI. The SRNC can then proceed with Handover Processing, as discussed, for example, by Alcatel-Lucent, et al., 3GPP TSG-RAN3 Meeting #73, R3-112026, “Macro To Small Cell, Metro Cell Hand-In,” Athens Greece, Aug. 22-26, 2011.        
For the sake of completeness, it shall be mentioned that once the relocation procedures are triggered, there can be several different types of signalling procedures that can be followed. According to the architecture shown in FIG. 2, mobility signalling towards a CSG cell can either go through the core network or CN (Iu based mobility) or, in case of CSG cell to CSG cell mobility, it can be routed via the Iurh interface without passing through the CN.
As shown in FIG. 2, each NodeB base station may be coupled to a respective radio network controller RNC over an Iub interface, and each radio network controller RNC may be coupled to a core network CN over a respective Iu interface. Moreover, each NodeB base station may support communications with wireless terminals UEs over a Uu air interface(s). While one NodeB base station and one radio network controller are shown by way of example, a radio access network (RAN) may include any number of such elements.
Home NodeB base stations HNB(s) may be coupled to core network CN through a security gateway and a Home NodeB gateway HNB GW. More particularly, each Home NodeB base station HNB may be coupled to HNB GW through the security gateway over a respective Iuh interface, and HNB GW may be coupled to core network CN over an Iu interface. Moreover, each HNB may support communications with wireless terminals UEs over a Uu air interface(s), and different HNB GWs may be coupled over Iurh interfaces.
Pre-Release 9 UEs may be unable to follow the procedures described with respect to FIG. 1. Such UEs will from now on be referred to as “legacy UEs”. For these UEs it may be very difficult to disambiguate an HNB target cell at the SRNC. In contrast, Release 9 and later UEs will be referred to as non-legacy UEs.
Legacy UEs can monitor three mutually exclusive categories of cells (and report their PSCs): active set (cells in soft handover or HO), monitored set (cells not in soft HO, but included in the UE's NCL or Neighbour Cell List); and detected set (cells detected by the UE, which are neither in the monitored set nor in the active set). The detected set reporting is applicable only to intra-frequency measurements in CELL_DCH (Cell Dedicated Channel) state for legacy UEs, but later releases also support inter-frequency measurement of detected cells. A UE needs to be able to report the measurements within: a) 200 ms for an already identified cell; b) 800 ms for a new cell in the monitored set; and c) 30 s for a new cell in the detected set (see, 3GPP TS 25.133 V10.4.0, “Requirements For Support Of Radio Resource Management,” Release 10, December 2011). The difference in the detection time requirements of cells in the detected set and the monitored set is due to the UE's usage of matched filter measurements to identify the cells, where the filter parameters might be configured for efficient detection of the monitored set only. However, there can be UE implementations where the detection time for both the monitored set and the detected set is quite comparable.
As explained above, even if a legacy UE is able to report detected set cell PSCs there may be more than one cell using a same PSC. Therefore, the SRNC may have difficulty unequivocally identifying a target cell corresponding to a PSC reported by legacy UEs.
A feature that a non-legacy UE may support is to be CSG-capable, namely being able to be configured for System Information (SI) acquisition for any detected cell. The latter is a measurement configuration that was standardised to allow UEs to detect and report System Information of femto cells not necessarily included in the SRNC Neighbour Cell List. As discussed above, there may be a disparity in the behaviour of Release 9 UEs and later releases versus pre-Release 9 UEs for cases of mobility to CSG cells.
Release 9 and later UEs may be configured to report a Proximity Indication flag, signalling that there is an accessible CSG cell in proximity. Further, these UEs may be configured to report, once the CSG cell in proximity is detected, the PSC, CGI, CSG ID and Membership Status of such CSG cell.
With the information above reported by the UE, the SRNC is able to uniquely identify the target cell to which the UE shall be handed over, and it is able to correctly construct mobility messages that can unequivocally trigger preparation of resources at the target before the UE relocates. Alternatively, mechanisms allowing mobility to target cells used for all UE releases is based on the SRNC broadcasting over the serving cell a list of neighbour cells, also known as the Neighbour Cell List (NCL). This list indicates scrambling codes of cells for which the SRNC has all the required information to initiate mobility. Once a UE reports one of the PSCs in the NCL, the RNC can immediately generate the mobility signalling towards the target RNC. However, it has to be noted that the NCL may have a very limited size. For intra-frequency cells, the NCL is limited to 31 entries and for inter-frequency cells it is limited to 32 entries. Therefore, it may not be feasible to include all the cells in a given neighbourhood (CSG cells, small cells, macro cells etc.) in the NCL.
Another relevant background technical aspect is the Automatic Neighbour Relation (ANR) function currently available in WCDMA systems.
The ANR function is enabled by means of keeping a Neighbour Relation Table (NRT) in the SRNC, which lists all the LTE, GSM and 3G cells that are neighbours of the cells served by the SRNC. The NRT is independent from the NCL, namely cells that are present in the NRT may not be present in the NCL. Different instances of the NRT may be kept for each cell served by the RNC.
By way of example (relating to mobility for 3G CSG cells), the case of 3G cells configured in the NRT will be considered.
If a 3G cell is configured in the NRT (Neighbour Relations Table), the following parameters shall be stored in the NRT [as specified in the reference 3GPP TS 25.484 V10.0.0, “Automatic Neighbour Relation (ANR) for UTRAN,” Stage 2, Release 10, 2011-07], and shall form the Neighbour Cell Identifier (NCI):                PLMN-Id,        Cell Identifier(C-ID), and        RNC-ID/Extended RNC-ID.        
Other pre-configured flags are present for each cell saved in the NRT. These pre-configured flags are listed below.                No Remove: If checked, the RNC shall not remove the Neighbour cell Relation from the NRT.        No HO (Hand Over): If checked, the Neighbour cell Relation shall not be used as a neighbour cell by Intra RNS (Radio Network Subsystem), Inter RNS or Inter RAT (Radio Access Technology) mobility functions in UTRAN (Universal Terrestrial Radio Access Network).        
Besides the parameters above, the SRNC managing the NRT may need to know the following parameters concerning each cell in the NRT:                LAC (Location Area Code) and RAC (Routing Area Code), and        frequency information (UARFCN or UTRA Absolute Radio Frequency Channel Number, BCCH ARFCN or Broadcast Control Channel ARFCN, EARFCN or E-UTRA ARFCN)        
As described in the reference 3GPP TS 25.484, V10.0.0 (cited above), the SRNC can update its NRT by means of measurements collected by UEs supporting ANR (Automatic Neighbour Relations) capabilities. These UEs will collect measurements about cells monitored while attached to a given serving cell. The UEs will report such measurements in logs whenever the network believes it is opportune for the UE to report such information. The RNC to which the ANR measurement log will be reported will be responsible to distribute relevant measurements to the RNCs that can benefit from it, e.g., to those RNC that can update their NRT by means of such measurements.
The logged measurements reported by the UE (and specified in the reference 3GPP TS 25.331 V10.3.1, cited above) also include the PSC of each reported cell. Therefore, even though the NRT specified in the reference 3GPP TS 25.484, V10.0.0 (cited above) does not include a PSC for each included cell, an RNC can associate a PSC from the UE reported measurements to each newly created entry of its NRT.
The NRT currently specified in the reference 3GPP TS 25.484, V10.0.0 (cited above) is used to identify target neighbour cells by means of PSC reporting by the UE. Namely, once the UE reports the PSC of a cell included in the NRT, the SRNC immediately retrieves all the details about that cell. Such details can be used to construct the mobility messages needed to relocate the UE to the target cell, without requiring the UE to perform any further measurement.
However, the current NRT may be unable to help in cases of legacy UEs relocating to cells not uniquely identifiable by their PSC. In fact, if a legacy UE reports the PSC of a target cell to its SRNC and if this PSC is present more than once in the NRT, the SRNC may have no means to disambiguate the target because legacy UEs cannot receive a measurement control configuration allowing reporting of System Information of the detected cell. In contrast, Release 9 UEs and later releases can be configured in such way as described in the reference 3GPP TS 25.367 V10.0.0 (cited above).
A legacy UE, although not able to be configured to report System Information of a cell for which the PSC has been detected, can report the PSC and the CPICH (Common Pilot Indicator Channel) signal strength of any cell in range, namely also of cells that are not included in the Neighbour Cell List (NCL) broadcast by the serving cell. The range of cells for which reported PSCs are not included in the NCL is called the “Detected Set Cells”.
In the reference Qualcomm Inc., et al., 3GPP TSG RAN WG3 #73bis, R3-112600, “Legacy UE Macro to HNB Active Hand-in,” Zhuhai, China, Oct. 10-14, 2011, and in the reference Alcatel-Lucent, et al., 3GPP TSG-RAN3 Meeting #73, R3-112026 (cited above), solutions to support mobility of legacy UEs to small cells in need of disambiguation are presented. Methods followed in both of these references may include the operations listed below.                1. Configuring a SRNC with information about which HNB GW (Gateway) corresponds to a given range of PSCs.        2. Configuring the target HNB GW with information concerning each pair of cells where one cell is associated to a HNB served by the HNB GW and the other is a cell not under the HNB GW domain, with this information including:                    (a) Frame timing difference between cell under HNB GW domain and neighbour cell;            (b) Cell ID of each cell under HNB GW domain, which is neighbouring the cell not under HNB GW domain; and            (c) Pilot channel information of each cell under HNB GW domain, i.e. Primary Scrambling Code, ARFCN, CPICH RSCP (Received Channel Power Code).                        3. Enhancing the SRNC to allow it to include in the mobility messages towards the target HNB GW the information listed above in bullets 2, 2(a), 2(b), and 2(c).        4. Enhancing the HNB GW to disambiguate the target cell on the basis of the information received from source RNC via mobility messages.        
Established NRT information in the RNC may be aligned with the OaM (Operations and Maintenance) system, as shown in FIG. 3. This also enables NRT information to be added from the OaM system to the NRT in the RNC. As shown in FIG. 3, each radio network controller RNC may include a neighbor relation table NRT and a radio resource control RRC cooperating with an ANR (automatic neighbor relation function), and neighbor relation NR information may be shared between different radio network controllers through OaM system.
The provisioning procedure of HNBs may be as described in the reference 3GPP TS 25.467 V10.2.0, “UTRAN Architecture For 3G Home Node B (HNB),” Stage 2, Release 10, 2011-07, illustrated by FIG. 4. First, a secure tunnel is established to the security gateway of FIG. 2. Second, the HNB establishes contact with and provides location information to the HNB Management System (HMS) (including location verification, GW discovery, and HNB provisioning using TR-069). Such location information may include measurements obtained from embedded UE functionality in the HNB. Third, reliable transport setup (SCTP) may be provided between the HNB and the HNB GW. Fourth, an HNB registration procedure may commence.
The table of FIG. 5 identifies the radio frequency and broadcast information that the HNB can provide to the HMS (HNB Management System). In other words, the table of FIG. 5 identifies macrocell information that the HNB provides to the HMS from measurements.
The OaM system may include logical components to manage each domain. Such domain managers include the Operation Support System to manage the UTRAN and the HNB Management System to manage the HNBs. The combined network domains are managed by the Network Management System (NMS) as illustrated by FIG. 6.
As discussed above, Pre-release 9 UEs may be unable to provide enough information to the SRNC to allow disambiguation of a target cell not included in the NCL broadcast by the serving cell.
Operations discussed above may have significant impact on current infrastructure because changes may be required both at RNCs and at HNB GWs. Moreover, these operations may contradict one of the basic assumptions of mobility in 3GPP systems, i.e., that the source RAN (Radio Access Network) is responsible to decide which cell is the target of the mobility procedure. Finally, these operations may not address the issue of legacy UEs, which may be unaware of closed subscribed group cells, and as such, may be unaware of whether they can access such closed cells or not, repeatedly attempting access to cells that are not accessible.
A legacy UE may at best report information about the PSCs of cells monitored in its neighbourhood and not included in the NCL, i.e. a legacy UE may report the Detected Set Cells. However, the SRNC might not know the system information of the cell corresponding to the reported PSC. Alternatively, the SRNC might know the details of a cell neighbouring the serving cell and having the same PSC reported by the UE, but it might be the case that there is another cell in the neighborhood sharing the same PSC. The SRNC may be unable to detect such duplication and/or to prevent/reduce relocation attempts towards an incorrect target(s).